1. Field of the Invention
This invention relates to the field of magnetic recording, and in particular to the duplication of recorded information from a magnetically recorded master medium to a slave magnetic medium.
2. Description Relative to the Prior Art
A variety of techniques are known in the art for the duplication of a recording from a master tape to a slave copy. The methods may be divided into two broad classes; the head-to-head duplication method wherein the master tape is played back on an appropriate reproducer and the resultant signal is used to drive a series of recorders which generate the slave copies, and the contact duplication methods wherein the master and slave media are placed in intimate contact and the transfer accomplished by means of an anhysteretic recording process or by a thermal recording process.
In anhysteretic processing, the master tape, which typically has a coercivity of three times that of the slave tape, is placed in intimate contact with the slave tape. The in-contact master and slave are transported through a decreasing amplitude alternating sign magnetic field which successively switches the magnetization of the distribution of magnetic particles of the slave between the two magnetic states of the particles. As the tapes traverse the field, and as the field amplitude decreases, the remanent magnetization of the slave assumes a value proportional to the magnetization of the master. The field, known as the bias field or transfer field, is not of great enough amplitude to substantially affect the magnetization of the master tape.
The thermal recording process utilizes CrO.sub.2 or other low Curie point magnetic particle tape as the slave. The CrO.sub.2 tape is heated to lower its coercivity while placed in contact with the master tape. The master tape and the slave are then transported together ( without the use of a transfer field), and as the CrO.sub.2 cools, its coercivity increases and its magnetization becomes fixed at a level determined by the signal on the master tape.
A detailed discussion of the above tape duplication methods may be found in the "Magnetic Recording Handbook", Mee and Daniel, McGraw-Hill Publishing Company, 1989, pp. 959-968.
For further consideration of the anhysteretic method, reference is made to FIG. 1, which schematically represents a typical anhysteretic duplication device known in the art. A master tape 10 and a slave tape 12 feed from supply reels 14,16 respectively. The tapes 10 and 12 are placed in intimate contact while they traverse a transfer field typically generated by a single gap magnetic head 18 driven by an a.c. generator 20. Considering the segments of the tapes 10,12 which are in contact as the tapes traverse the field of the head 18, it will be seen that the segments in contact experience a monotonically diminishing transfer field as they move away from the head 18. As described above, this results in the master's recorded information being imprinted onto the slave. The master tape 10 and the duplicated slave tape 12 are then wound onto takeup reels 22,24 respectively.
It is known in the art that the in-contact segments of the master and slave tapes 10,12 must experience several cycles of the transfer field in order to effect the duplication. It will therefore be appreciated that if the speed of the tapes 10,12 is increased in an attempt to increase the duplication rate, the frequency of the generator source 20 must correspondingly increase. This requires increased currents from the source generator 20 to drive circuit and stray capacitances, and also results in increased power dissipation in the head 18 due to higher frequency losses in the head core material.
The above duplication speed problem was recognized in U.S. Pat. No. 3,519,760 entitled "Magnetic Duplicating Apparatus Using a Multiple Gap D.C. Head", issued in the name of E. T. Hatley. This patent discloses a multi-gap d.c. excited head, or a multi-permanent-magnet head, which generates a spatially varying, but time invariant magnetic field to serve as a transfer field. In duplicating apparatus utilizing such a transfer field, each segment of the in-contact master and slave tapes, while traversing the magnetic transfer field, will experience magnetization reversals equal to twice the number of spatial cycles of the magnetic field generated by the head regardless of tape speed. The use of a spatially extensive, time-invariant field accordingly overcomes the duplication speed limitation inherent in the use of the a.c. driven, single gap head considered in the prior art apparatus of FIG. 1.
There are, however, other problems incident to tape duplication through the use of the apparatus incorporating the multiple gap head of the above referenced patent. It is known in the art that slippage between the in-contact master and slave tapes as they pass through the transfer field is severely detrimental to duplication of the short-wavelength signals. (See, for example, "Magnetic Recording Handbook", Mee and Daniel, supra, page 964.) U.S. Pat. No. 3,519,760 teaches the use of a spring-loaded pressure pad which obviates such slippage by forcing the master and slave firmly together against the planar face of the multiple gap transfer head as they move past the head. This solution, however, limits the tape speed, and resultantly the duplication speed, due to the heating of the tapes by friction at the interface with both the pressure pad and the multi-gap head. The temperature rise due to such heating detrimentally alters both the magnetic and structural properties of the tapes as well as causes excessive wear of the transfer-field head.
The apparatus disclosed in U.S. Pat. No. 3,519,760 also suffers from the disadvantage in that the magnetic transfer head has an even number of alternating magnetic pole elements. This configuration causes the respective fields of the first and last pole elements to be in opposite directions, which results in stray magnetic fields as well as magnetic coupling between the first and last elements via a flux path through the tapes. The stray magnetic fields are potentially harmful to magnetic equipment and media whereas the magnetic coupling gives rise to a DC remanent field in the slave tape that causes harmonic distortion in the duplicated recording.